The government has been urged to explain to schools how they can ensure that classrooms are well ventilated without forcing teachers and pupils to work in uncomfortably cold conditions.
Mary Bousted, the joint general secretary of the NEU teaching union, said the Department for Education’s guidance on keeping schools ventilated but warm enough to work in during the winter was "lacking".
Her comments come as teachers are already highlighting how cold their schools are at the end of September as doors and windows are kept open in an attempt to curb the spread of Covid-19.
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Teachers say they are working in "miserable" temperatures, with numb fingers, wearing scarves and coats all day.
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And there are concerns that this issue will become much worse over the winter.
Dr Bousted said: “This is an area where government guidance is lacking in terms of the balance to be struck between optimising the amount of fresh air and balancing this with thermal comfort.
“With the colder months approaching, the DfE needs to provide schools with urgent practical advice on how to ensure that that safe ventilation is maintained whilst at the same time ensuring a comfortable working temperature.”
Scores of teachers have taken to social media in the past week to highlight how cold their schools have become, with many suggesting that pupils may need to learn in their coats.
The government’s guidance on the full return of schools says: “Once the school is in operation, it is important to ensure good ventilation and maximising this wherever possible; for example, opening windows and propping open doors, as long as they are not fire doors, where safe to do so (bearing in mind safeguarding in particular).”
The NEU’s website advises school staff that their workplace temperature should be reasonable.
The union’s guidance, which predates the Covid-19 pandemic, says, under the Workplace (Health, Safety and Welfare) Regulations 1992, "which apply to all workplaces, including schools", temperatures should be “reasonable”.
This, the guidance says, is defined as "normally at least 16C" (60F) during "the length of time people are likely to be there".
The DfE was contacted for comment.